Method of stripping sheets.



J. MQFETRIDGE, METHOD OF STRIPPIN/G SHEETS.

APPLIGATION FILED OCT. 28, 1912.

5 V Patented May27, 1913.

ZSHEETS-SHEET 1.

WITNESSES INVENTOR 9 W ww ozgmawm J. M FETRIDGE. METHOD OF STRIPPING SHEETS.

APPLIGATION FILED 001 .28, 1912.

1,063,054. Patented May 27, 1913.

2 SHEETS-SHEET 2.

WIETNESSES INVENTQR UNITED srarns PATENT OFFICE.

JOSEPH MCFETRIDGE, OF VANDERGRIIFT, PENNSYLVANIA, ASSIGNOR T AMERICAN SHEET & TIN PLATE COMPANY, OF PITTSBURGH, PENNSYLVANIA, A GORPORA TIO'N OF JERSEY.

METHOD OF STRIPPING' SHEETS.

To all whom. it may concern Be it known that I, Josarn MGFETRIDGE, a citizen of the United States, residing at Vande-rgritt, in the county of Westmoreland and State of Pennsylvania, have invented a new and useful Method of Stripping Sheets, of which the following is a. specification.

My invention relates to removal of the zinc coating from galvanized materials, and more particularly relates to the stripping 0t defective galvanized sheets in preparing such sheets for re-galvanizing and the recovery of the removed zinc as a. product.

adapted for use in further galvanizing operations and for other purpos s.

I-leretofore, in stripping detective galvanized sheets it has been the practice to place the sheets in a bath formed of a mixture of ferrous salts and free acid, this acid-ferrous liquor being a byproduct of the process of pickling sheet steel, and known in the shops as spent pickle liquor. Spent pickle liquor contains variable quantities of free acid, depending on the practice of the shop, and when the free acid has all been neutralized by the addition of galvanized sheets, the bath, then consisting of a mixture of ferrous salts and zinc salts, is discarded as waste product and the zinc value lost to the process. In

shops where partly spent pickle is not avail able for stripping detective galvanized sheets, it is common practice to place. the sheets in 'a bath of weak acid, in which the sheets are allowed to reinaiu until the zinc and zinc-iron alloy is removed therefrom and the surface. of the. base'material is uncovered. lhis practice. gives a moderately dilute solution of zinc salts high in ferrous salts and has no comn'icrcial value.

One object of my invention is to provide. a novel process otstrip 'iiug galvanized materials and defective galvanized sheets whereby a. minimum amount of ironis dissolved during the stripping operation and by which the. solution or liquor used in the stripping operation is maintained at full strength without being concentrated.

Another ol'iject otmy invention is to provide. an improved process ot stripping the galvanized materials and conserving thev zinc values whereby the. zinc coating of the galvanized materials is recovered as a prod not adapted for use In further galvanizing Specification of Letters Eatent.

Application filed October as. 1912.

Patented May 27, 1913.

Serial No. 728,169.

operations and for other commercial purposes.

. A further object of this invention is to provide an improved method of stripping galvanized sheets whereby the amount of iron dissolved during the stripping operations is greatly reduced and to a considerable extent. prevented.

Referring to the accompanying drawings forming part of this specification, Figural is a diagrammatic plan showing the construction and arrangement of apparatus adapted for use in carrying out my improved process. Fig. 2 is an elevation, partly in section, of the apparatus shown in Fig. 1.

In the drawings 2 designates 'a-tank or vessel for storing strong acid having a valve controlled discharge outlet 3 arranged to deliver acid into the pickling vat 4:, which is located below the tank 2 so as to permit the acid to flow into the at 4 by gravity. The vat at is provided with valve controlled discharge outlets 5 and 6 arranged to deliver partly neutralized liquor from the vat 4 into the vats 7 and 8 in which the second step of my improved process is carried out. The vats 7 and 8 have valve controlled outlets 9 and 10 discharging into a sump or well 11 and a steam siphon 12 is employed as a convenient means for lifting or elevating the solution from the well 11 into the storage tank 13 which is elevated so as to discharge through its valve controlled outlet 14 into the open upper end of the tower 15 in which the solution or liquor from the tank 13 is oxidized. 'lhe tower 15 is packed with material such as quartz 16 which is not affected by the acid solution of zinc salts as it trickles or flows downwardly therethrough and the lower end of the tower 15 is provided with an inlet 17 through which a supply of oxidizing gas,.su ch as chlorin gas, is delivered into the tower at its lower end so as to rise upwardly through the quartz or other material 16 with which the tower is packed. The liquor or solution treated or oxidized in the tower 1:) is discharged by gravity into one or the other of the storage tanks 19 and i0 and the oxidized liquor is treated in the tanks 15) and with zinc cariron insoluble. The material from the tanks 19 and 20 after being treated in this manner is delivered by the pump 21 through the conduit 22- into the chamber 223 in which it. is decanted and filtered and the resulting commercially pure zinc solution is delivered from the cl amber or vessel 255 by the con duit 21- into the storage tanks 25 and 26.

In carrying out the steps ot ni v imprcned process of stripping det'ective galvanized sheets, or other materials truth which the zinc coating is to be removed, the galvanized materials are placed in the vat l which co tains a bath composed of zinc ehlorid or zinc sulfate to which two to three per cent. o't free acid has been added from the. acid tank 2, the galvanized materials being allowed to remain in the vat t until the zinc-iron alloy becomes exposed. The. so-treated ii'iatcrials are then removed from the vat l to one of the vats 7 and 8. galvanized materials are treated in the vat at until the zinc coating is su stantially removed, then being transferred "from the vat 4 into one or the other of the vats 7 and 8 containing liquor of greater s]. ecitic gravity (in degrees Baum) and less free acid to be further treated to remove the zinc-iron alloy now forming the surfaces of the partly stri ped materials. When the acid content of tie bath or solution in the vat 4 has been reduced from between two and three per cent. to between one-halt": and one per cent, a portion of the bath is withd awn from this vat into one or the other of the vats 7 and S to form the bath in such vats. The quantity of solution transferred from the vat 4- to the vats 7 or 8 should equal in volume the amount; of acid necessary to restore the acidity in the solution in the vat 4. to its original strength of from two to three per cent. free acid. By following this procedure, the bath is of uniform strength and acidity and the action of the acid on zinc of the galvanized sheet is slow enough to be within control of the operator so that a minimum quantity of the base metal is dissolved and the finished product is of uniform strength and composition. The partly stripped materials as removed from time to time from the vat 4 are placed in one or the other of the vats 7 or 8 where they remain until the zinc coating and zinc-iron alloy has been entirely removed from the materials being treated or stripped and the solution with which the vats 7 and 8 are kept filled is used until the acid is satisfied and the bath has become entirely neutralized. As the solution in the vat 4 is maintained with an acid content ranging from between two and three per cent. When freshly replenished to one-half to one per cent. at the time the solution is withdrawn into one of the vats 7 or 8, the zinc coating will be removed more rapidly in the vat 1 Successive quantities ofthe second step as fast as the first step of the process is carried out. Instead of using two vats 7 and S, as shown, one large vat having a capacity equal to that of both vat 7 and 8 may be employed. The so-stripped plates when removed from the vats 7 and R are then washed to remove. any reu'iaining solution therefrom and are then in readiness to be again galvanized. Vhen the solution in the vats 7 and 8 through continued use becomes neutralized, it is discharged into the well 11 from which it is delivered by the siphon 12 into the storage tank 13. which provides a convenient means by which solution in the tanks 7 and 8 may be removed therefrom at irregular intervals while a con stant supply of solution is obtained for treatment in the tower 15. The solution in the tank 13 is discharged into the top of the tower 15 and trickles or flows downwardly through the material with which this tower is packed, and in passing through the tower the solution is brought into intimate contact with the upwardly ascei'iding column of oxidizing gases which are introduced at the bottom of the tower and is oxidized, contact of the solutionwith the ascending gases cl'ianging the iron in the liquor from the terrous to ferric salts. The oxidized liquor is discharged by gravity from the bottom of the tower and passes into one or the other,

of the tanks 19 and 20 in which it is treated with zinc carbonate so as to precipitate the iron which is now in the ferric form. This treatment changes the iron into an insoluble form and the treated liquor is delivered from the tanks 19 and 20 to the receiver in which it is decanted and filtered so as to leave pure Zinc salts. The zinc salts are then discharged into the storage tanks 25 and 26 from which they are withdrawn when required.

The advantage of my invention Will be apparent to those skilled in the art. Instead of the zinc removed in the stripping operations being allowed to go to waste, as l'ieretotore, itis recovered in a form in which it may be readily sold.

By first partly stripping a given quantity of sheets in an acid solution of zinc chlorid or zinc sulfate and then completing the stripping operations in a similar solution of greater specific gravity and less free acid and by preventing the first solution-from L I becoming neutralized or its acid content be than 1n the bath maintained in i 111g reduced below a certain point, and using rapidly removing the coating in a strong acid solution and then completing the operation with a weaker solution on a larger quantity of the materials at a slower rate of speed, the amount of iron dissolved in the stripping operations is greatly lessened and largely prevented.

Modifications in the steps of my method may be made without departing from my invention as defined in the claims. The

' amount of acid in the first bath and the relative amount of acid in the solution forming the second bathmay be-varied and other changes may be made.

I claim: I

1. The method of stripping galvanized material which consists in placing the ma-' terial'in a bath of a zinc salt and acid to partly strip the material, removing the partly stripped material therefrom to a sec,- ond bath to remove the zinc-iron alloy and complete the stripping operation, removing portions of partly neutralized solution from the first bath to form the second bath,

and making additions of acid to the first bath to restore its strength.

2. The method of stripping galvanized material and conserving the'zinc values which consistsin stripping the material in .a bath of a zinc salt and acid until the coating is partly removed, removing the partly stripped materials to a second bath of zinc chlorid and acid and completing the stripping operation therein, using the second bath until neutralized and withdrawing .portions of partly neutralized solution from the first bath to replace neutralized solu-' tion in the second bath.

3. The method of stripping metallic coated material which consists in placing the material in an acid bath of from two to three per cent. free acid to partly strip the coating from the material, thereby partially neutralizing the acid in said bath, removing second bath to complete the stripping operation, and removing portions of the partly neutralized solution forming the first bath to form the second bath when the acid conthe partly stripped material therefrom to a tent of the first bath becomes reduced to between one-half and one er cent.

4. The method of stripping metallic coated material which consists in placing the material in an acid bath of from two to three per cent. free acid to partly strip the coating from the material, thereby partially neutralizing the acid in said bath, removing the partly stripped material therefrom to a second bath to complete the stripping opera tion, and removing portions of the partly neutralized solution forming the first bath to form the second bath when the acid con: tent of the first bath becomes reduced tobetween one-half and one per cent. and making additions of acid to the first bath to re store its strength to between two and three my hand.

JOSEPH MCFETRIDGE.

Witnesses:

A. H. BEALE, R. G. Secret. 

